How do you judge how much a scientific study or academic article has been used? You can see how frequently it’s cited, but since researchers and academics read and are influenced by plenty of things that don’t get formally checked in their work, that doesn’t tell the whole story. Researcher Philip Davis is trying to provide some new answers to that question by taking a look at ‘usage half-life,’ in an effort to learn more about the academic publishing life cycle. Usage half-life is an estimate of how long it takes group of articles published in an online journal to reach one half of the number of downloads that they will ever have. Davis hopes the metric can contribute to the continuing conversation surrounding journal access, by giving everyone from librarians to policy makers a reliable, evidence-based tool for determining how journals are used by students, professors, and researchers. “The formation of good science policy should be guided by scientific evidence,” Davis told Library Journal. “Not speculation, and certainly not wishful thinking.” Similar means for measuring citation of a given article have been around for years, said Davis, but it’s only recently that tracking of article downloads has made it possible to see not only what researchers are citing in their papers, but what they and others are reading as well. Davis admits that showing how many times an article has been downloaded does not show how many times it has been actually read, but said that the download number is at least a good place to start. Davis’ study, which was funded by the American Association of Publishers (AAP) Professional & Scholarly Publishing division, found that only three percent of journals hit their half-life mark in less than 12 months. Engineering journals were the mostly likely to have brief half-lives, with 6 percent achieving their half-life in less than a year, while only one percent of life science journals hit their half-life in that time span. The shortest half-lives Davis recorded belonged to journals covering health and medical science, where articles reached their half-life in an average of between two and three years. Journals with the longest usage half-life tended to cover disciplines like physics and mathematics, as well as the humanities. Most journals seem to hit their half-life between two and four years. David Crotty, senior editor at Oxford University Press, argued on the Scholarly Kitchen blog that this shows the 12 months of journal exclusivity given to federally funded research before it is made publicly available under last year’s Office of Science and Technology (OSTP) mandate—and a hallmark of the Green OA model—may be out of step with how long journal articles retain their impact. (AAP, the study's sponsor, has long lobbied for longer embargo periods; it applauded the OSTP for making the 12 months a guideline and empowering agencies to extend it.)  The 12-month embargo period was based on data from NIH funded papers released to PubMed central, which Crotty says may not be the most definitive sample according to this study. “The new study, however, suggests that the NIH experience may have been a poor choice for a starting point,” writes Crotty. “Clearly the evidence shows that by far, Health Science journals have the shortest article half-lives. The material being deposited in PubMed Central is, therefore, an outlier population, and many not set an appropriate standard for other fields.” The idea that making material open-access hurts publishers, though, isn’t a universally accepted one. A 2006 survey of librarians by the Association of Learned and Professional Society Publishers found that the availability of studies via OA archives was not a leading factor in deciding to cancel journal subscriptions, with librarians ranking it far behind more pertinent concerns like price and the needs of faculty. Crotty worries that leaving journals with just a year of exclusivity could kneecap the publishing industry financially. If most journal articles are still in their relative youth, as far as interest is concerned, after just a year, he argues, losing their exclusivity then could cause readers to jump ship, waiting until research is released into the open rather than paying for subscriptions. Others, though, aren’t convinced that argument holds water. “At best, the connection between so-called article half-life and canceled subscriptions is based on a highly speculative argument that has yet even to be made,” said Kevin L. Smith, Director of the Office of Copyright and Scholarly Communication at Duke University and a regular columnist for Library Journal, as well as a member of the steering committee of the Scholarly Publishing and Academic Resources Coalition (SPARC) of the Association of Research Libraries (ARL) Even if the concern is valid, Smith points out, the point of the OSTP mandate is not to be financially beneficial for publishers, but to ensure that federally funded research is openly available to the public. “It may still be good policy to insist on six month embargoes even if we had evidence that this would have a negative economic impact on [some] publishers,” Smith told Library Journal. “Government agencies that fund research simply are not obligated to protect the existing monopoly on the dissemination of scholarship at the expense of the public interest.”